A television image or picture is defined by a certain number of parameters that represent the spatial and temporal analysis of this image (in particular, the number of images per second, lines per image and pixels per line).
The passband necessary to transmit an HDTV image may attain values of the order of 100 MHz. Thus, to transmit these images on passband channels with reduced width (for example, satellite channels with a band of the order of about ten megahertz), it is necessary to apply passband reduction techniques to these images.
The passband of the signal to be transmitted can be reduced by using adaptive techniques for filtering the high definition image. This filtered high definition image then has a reduced basic passband and may be re-sampled at lower frequencies for transmission. As the filterings are selected in an adaptive way at the transmitter, assistance data are transmitted to the decoder so that it recognizes the processing chosen at the encoder.
These items of assistance data therefore consist of an indicator of the chosen processing, but may also include motion vectors when motion compensation is used.
The throughput rate of the assistance data in the transmission channel is limited to about 1 Mbit/s.
The large quantity of information that these items of data represent makes it necessary to use throughput reduction techniques.
Any throughput reduction method may be described in terms of two consecutive steps:
1. Reduction of the flow of information to be transmitted; PA0 2. Encoding (or representation) of the information for transmission. PA0 the encoding of the information is reversible. This step consists in placing the information in the form of a sequence of bits which represents the information completely. PA0 the process of compression of the image signal consisting in the segmentation of the image into substantially homogeneous blocs of variable sizes i.times.i for which all the pixels may be defined collectively, the possible sizes being capable of classification according to levels of segmentation, from a minimal size of elementary blocks, each level corresponding to the regrouping of four lower level adjacent blocks, PA0 said compression process comprising a selection step, for each elementary image block, of an optimal processing among several concurrent compression processing operations achieved in parallel, the processing operations comprising notably, on the one hand, at least one compression processing operation by image filtering and, on the other hand, at least one compression processing operation by motion compensation, PA0 a method wherein said assistance data comprise, essentially, for each block, firstly a piece of information on the choice of processing associated with a piece of information on the segmentation of the block, and secondly, in case of a choice of compression processing by motion compensation for the current block, a piece of motion vector information representative of said block associated with a piece of information on segmentation of the block, PA0 and wherein, in a first stage, a hierarchical encoding in increasing order is achieved, on at least two levels of segmentation, for the information on choice of processing operation and, in a second stage, in case of a choice of processing with motion compensation for a non-elementary block identified in the first stage, a hierarchical encoding in increasing order, on at least two levels of segmentation, for the motion information, PA0 and wherein said assistance data are generated under the control of sequencing means locked into a pre-determined image scanning procedure, in selectively generating pieces of segmentation information only for the non-elementary blocks and in selectively generating pieces of information on motion vector choice only during the occurrence of a block with optimal processing by motion compensation. PA0 for each processing, a bit on processing choice associated with each elementary block of the image is positioned, said bit taking the information on validity or non-validity of the corresponding processing for the associated block; PA0 the positions taken by the processing choice bits at the upper level of segmentation are explored iteratively, and the existence of a block with homogeneous processing is validated at said upper level in case of homogeneity of the processing choice bits for all the blocks forming said upper level of segmentation. PA0 the value of the motion vector associated with a first elementary block is memorized, and a counter associated with said motion vector is initialized; PA0 the value of the motion vector associated with the higher level of segmentation is explored iteratively, in incrementing said counter at each occurrence of said motion vector, and a validation is done of the existence of a homogeneous motion block at said upper level of segmentation in the event of homogeneous processing by motion compensation at said upper level of segmentation and when said counter marks the number of blocks comprising said upper level of segmentation. PA0 and all said first and/or second bits are positioned systematically, in parallel for each level of segmentation, and independently of the value of the bit corresponding to the lower level of segmentation. PA0 a first word identifying the processing choice segmentation in said superblock; PA0 at least one second word identifying said processing associated with each homogeneous i.times.i block of a maximal level; PA0 a third word identifying the segmentation of motion in said superblock; PA0 at least one fourth word identifying the motion vector at each i.times.i block of maximum level with processing by motion compensation and homogeneous motion vector.
The two steps have the following properties:
the reduction of the data is irreversible for it modifies the original signal. In this case, the reduction of information may consist in making a choice concerning a set of pixels rather than each pixel;
These two steps may produce a reduction in the throughput rate.
A French patent application has been filed for a method to reduce the throughput rate of a sequence of data for assistance in the reconstruction of an electronic image using a sub-sampling signal (File Application No. 8802650 dated 2nd March 1988 on behalf of the present Applicants).
However, the invention described therein in no way resolves the problem of the encoding of information for transmission.